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  深入理解MVCC与BufferPool缓存机制
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  <time datetime="2022-05-27T15:16:23.000Z" itemprop="datePublished">2022-05-27</time>
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  <p id="hitokoto">正在加载一言...</p>

<h2 id="1、MVCC"><a href="#1、MVCC" class="headerlink" title="1、MVCC"></a>1、MVCC</h2><p><strong>概念：</strong></p>
<p>MVCC：<strong>Multi-Version Concurrency Control</strong>，多版本并发控制</p>
<p>作用：读已提交和可重复读隔离级别下，对一行数据的读和写两个操作默认是不会通过加锁互斥来保证隔离性，避免了频繁加锁互斥，保证并发事务的安全性。</p>
<p><strong>undo日志版本链与read view机制</strong></p>
<p>undo日志版本链是指一行数据被多个事务依次修改过后，在每个事务修改完后，Mysql会保留修改前的数据undo回滚日志，并且用两个隐藏字段trx_id和roll_pointer把这些undo日志串联起来形成一个历史记录版本链</p>
<p>具体对于MVCC的理解，详见下图的示例</p>
<p><strong>需要注意的点</strong></p>
<p>读已提交是每次执行select就生成一次一致性试图，可重复读只在第一次select的时候生成。</p>
<p>在可重复读隔离级别下，在一个事务中，一致性试图实在第一次执行select语句的时候生成的，除非有其它增删改操作，否则一致性试图不会变。同时无论第一次执行的是哪个表的select语句，其它表的一致性试图都不会变，即开始执行的select语句，会生成当前所有表的一致性视图；</p>
<p>示例如下：</p>
<p>（1）begin，执行actor表的select语句</p>
<p><img src="/2022/05/27/00-02-06-%E6%B7%B1%E5%85%A5%E7%90%86%E8%A7%A3MVCC%E4%B8%8EBufferPool%E7%BC%93%E5%AD%98%E6%9C%BA%E5%88%B6/image-20220528135240234.png" alt="image-20220528135240234"></p>
<p>（2）查询account表：</p>
<p><img src="/2022/05/27/00-02-06-%E6%B7%B1%E5%85%A5%E7%90%86%E8%A7%A3MVCC%E4%B8%8EBufferPool%E7%BC%93%E5%AD%98%E6%9C%BA%E5%88%B6/image-20220528135303794.png" alt="image-20220528135303794"></p>
<p>（3）更改account表的数据，添加一条</p>
<p><img src="/2022/05/27/00-02-06-%E6%B7%B1%E5%85%A5%E7%90%86%E8%A7%A3MVCC%E4%B8%8EBufferPool%E7%BC%93%E5%AD%98%E6%9C%BA%E5%88%B6/image-20220528135325995.png" alt="image-20220528135325995"></p>
<p>（4）再次查询account表数据，结果没变</p>
<p><img src="/2022/05/27/00-02-06-%E6%B7%B1%E5%85%A5%E7%90%86%E8%A7%A3MVCC%E4%B8%8EBufferPool%E7%BC%93%E5%AD%98%E6%9C%BA%E5%88%B6/image-20220528135359550.png" alt="image-20220528135359550"></p>
<p><a target="_blank" rel="noopener" href="https://www.processon.com/view/link/6291a3ea637689545dd6d4ae">MVCC示例图</a></p>
<h2 id="2、Innodb引擎SQL执行的BufferPool缓存机制"><a href="#2、Innodb引擎SQL执行的BufferPool缓存机制" class="headerlink" title="2、Innodb引擎SQL执行的BufferPool缓存机制"></a>2、<strong>Innodb引擎SQL执行的BufferPool缓存机制</strong></h2><p><a target="_blank" rel="noopener" href="https://www.processon.com/view/link/6291e2e57d9c085adb719a5f">Innodb引擎底层结构</a></p>
<p><img src="/2022/05/27/00-02-06-%E6%B7%B1%E5%85%A5%E7%90%86%E8%A7%A3MVCC%E4%B8%8EBufferPool%E7%BC%93%E5%AD%98%E6%9C%BA%E5%88%B6/image-20220528165311124.png" alt="image-20220528165311124"></p>
<p><strong>注：</strong></p>
<p><strong>上述步骤说明：</strong></p>
<p><strong>（1）、先按页查出老的数据所在页数据，</strong></p>
<p><strong>（2）、将老数据存储到undo 日志文件中，便于回滚</strong></p>
<p><strong>（3）、将内存数据（缓存数据）更新为新值，即zhuge-&gt;zhuge666</strong></p>
<p><strong>（4）、写Redo Log（重做日志），有缓存机制，批量写入</strong></p>
<p><strong>（5）、准备提交事务，redo log日志写入磁盘</strong></p>
<p><strong>（6）、准备提交事务，binlog日志写入磁盘</strong></p>
<p><strong>（7）、binlog日志写入磁盘完毕后，写入commit标记到redo日志文件里面，然后真正的提交事务；此步目的是为了保证redo日志和binlog日志数据一致；此步做完，客户端任务事务已经执行成功了，实际上数据可能还没有写入磁盘</strong></p>
<p><strong>（8）、随机写入磁盘，以page为单位写入，此步做完，磁盘里面的数据就变成了zhuge666。可能在执行此步前，mysql宕机了，可以通过redo log日志恢复数据</strong></p>
<p><strong>redo Log是什么？</strong></p>
<p><strong>是重做日志，用来恢复buffer pool里面的数据</strong></p>
<p><strong>bin Log是什么？</strong></p>
<p><strong>恢复磁盘数据</strong></p>
<p><strong>为什么Mysql不能直接更新磁盘上的数据而且设置这么一套复杂的机制来执行SQL了？</strong></p>
<p>因为来一个请求就直接对磁盘文件进行随机读写，然后更新磁盘文件里的数据性能可能相当差。</p>
<p>因为磁盘随机读写的性能是非常差的，所以直接更新磁盘文件是不能让数据库抗住很高并发的。</p>
<p>Mysql这套机制看起来复杂，但它可以保证每个更新请求都是<strong>更新内存BufferPool</strong>，然后<strong>顺序写日志文件</strong>，同时还能保证各种异常情况下的数据一致性。</p>
<p>更新内存的性能是极高的，然后顺序写磁盘上的日志文件的性能也是非常高的，要远高于随机读写磁盘文件。</p>
<p>正是通过这套机制，才能让我们的MySQL数据库在较高配置的机器上每秒可以抗下几干甚至上万的读写请求。</p>
<p>通过这样一套机制，既能保证数据不丢失，也能保证高效率</p>
<p>针对数据库磁盘数据的写是随机写，针对redo log是顺序写</p>
<p>为什么数据库磁盘数据文件不能顺序io，而redo log日志文件可以顺序io?</p>
<p>数据库磁盘文件会涉及到数据段额删除，可能一段顺序io中间一部分会被删除掉，插入其它的数据；而redo log文件只会顺序地在末尾新增，不会删除</p>
 
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                <!-- Preloader demo http://codepen.io/dimsemenov/pen/yyBWoR -->
                <!-- element will get class pswp__preloader--active when preloader is running -->
                <div class="pswp__preloader">
                    <div class="pswp__preloader__icn">
                        <div class="pswp__preloader__cut">
                            <div class="pswp__preloader__donut"></div>
                        </div>
                    </div>
                </div>
            </div>

            <div class="pswp__share-modal pswp__share-modal--hidden pswp__single-tap">
                <div class="pswp__share-tooltip"></div>
            </div>

            <button class="pswp__button pswp__button--arrow--left" title="Previous (arrow left)">
            </button>

            <button class="pswp__button pswp__button--arrow--right" title="Next (arrow right)">
            </button>

            <div class="pswp__caption">
                <div class="pswp__caption__center"></div>
            </div>

        </div>

    </div>

</div>

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe.min.css">
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/default-skin/default-skin.min.css">
<script src="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe-ui-default.min.js"></script>

<script>
    function viewer_init() {
        let pswpElement = document.querySelectorAll('.pswp')[0];
        let $imgArr = document.querySelectorAll(('.article-entry img:not(.reward-img)'))

        $imgArr.forEach(($em, i) => {
            $em.onclick = () => {
                // slider展开状态
                // todo: 这样不好，后面改成状态
                if (document.querySelector('.left-col.show')) return
                let items = []
                $imgArr.forEach(($em2, i2) => {
                    let img = $em2.getAttribute('data-idx', i2)
                    let src = $em2.getAttribute('data-target') || $em2.getAttribute('src')
                    let title = $em2.getAttribute('alt')
                    // 获得原图尺寸
                    const image = new Image()
                    image.src = src
                    items.push({
                        src: src,
                        w: image.width || $em2.width,
                        h: image.height || $em2.height,
                        title: title
                    })
                })
                var gallery = new PhotoSwipe(pswpElement, PhotoSwipeUI_Default, items, {
                    index: parseInt(i)
                });
                gallery.init()
            }
        })
    }
    viewer_init()
</script> 
<!-- MathJax -->

<!-- Katex -->

<!-- busuanzi  -->
 
<script src="/js/busuanzi-2.3.pure.min.js"></script>
 
<!-- ClickLove -->

<!-- ClickBoom1 -->

<!-- ClickBoom2 -->

<!-- CodeCopy -->
 
<link rel="stylesheet" href="/css/clipboard.css">
 <script src="https://cdn.jsdelivr.net/npm/clipboard@2/dist/clipboard.min.js"></script>
<script>
  function wait(callback, seconds) {
    var timelag = null;
    timelag = window.setTimeout(callback, seconds);
  }
  !function (e, t, a) {
    var initCopyCode = function(){
      var copyHtml = '';
      copyHtml += '<button class="btn-copy" data-clipboard-snippet="">';
      copyHtml += '<i class="ri-file-copy-2-line"></i><span>COPY</span>';
      copyHtml += '</button>';
      $(".highlight .code pre").before(copyHtml);
      $(".article pre code").before(copyHtml);
      var clipboard = new ClipboardJS('.btn-copy', {
        target: function(trigger) {
          return trigger.nextElementSibling;
        }
      });
      clipboard.on('success', function(e) {
        let $btn = $(e.trigger);
        $btn.addClass('copied');
        let $icon = $($btn.find('i'));
        $icon.removeClass('ri-file-copy-2-line');
        $icon.addClass('ri-checkbox-circle-line');
        let $span = $($btn.find('span'));
        $span[0].innerText = 'COPIED';
        
        wait(function () { // 等待两秒钟后恢复
          $icon.removeClass('ri-checkbox-circle-line');
          $icon.addClass('ri-file-copy-2-line');
          $span[0].innerText = 'COPY';
        }, 2000);
      });
      clipboard.on('error', function(e) {
        e.clearSelection();
        let $btn = $(e.trigger);
        $btn.addClass('copy-failed');
        let $icon = $($btn.find('i'));
        $icon.removeClass('ri-file-copy-2-line');
        $icon.addClass('ri-time-line');
        let $span = $($btn.find('span'));
        $span[0].innerText = 'COPY FAILED';
        
        wait(function () { // 等待两秒钟后恢复
          $icon.removeClass('ri-time-line');
          $icon.addClass('ri-file-copy-2-line');
          $span[0].innerText = 'COPY';
        }, 2000);
      });
    }
    initCopyCode();
  }(window, document);
</script>
 
<!-- CanvasBackground -->
 
<script src="/js/dz.js"></script>
 
<script>
  if (window.mermaid) {
    mermaid.initialize({ theme: "forest" });
  }
</script>


    
    

  </div>
<script src="/live2dw/lib/L2Dwidget.min.js?094cbace49a39548bed64abff5988b05"></script><script>L2Dwidget.init({"log":false,"pluginJsPath":"lib/","pluginModelPath":"assets/","pluginRootPath":"live2dw/","tagMode":false});</script></body>

</html>